In the aircraft industry, it has been generally recognized that one way of improving fuel efficiency of aircraft is to reduce the structural weight of airplanes. In reducing the structural weight of airplanes, aluminum alloys have been developed which have high strength to weight ratios along with high levels of fracture toughness, fatigue resistance and corrosion resistance.
One family of aluminum alloys typically used in commercial aircraft application is the Aluminum Association 2000 series of registered alloys.
Further improvements have been recognized in the prior art concerning the 2000 series aluminum alloys relating to improved fracture toughness and fatigue resistance by careful control of the processing steps during aluminum alloy plate manufacture. U.S. Pat. No. 4,294,625 to Hyatt et al. describes aluminum alloys, more particularly 2000 series aluminum alloys characterized by high strength, very high fatigue resistance and very high fracture toughness. The patent to Hyatt et al. discloses a method for producing the plate product from an aluminum alloy having high toughness comprising casting the alloy into a body and hot working the body to form a plate product. The plate product is then solution heat treated such that the maximum amount of copper in the alloy is taken into solid solution. Following the solution heat treating step, the plate product is quenched, pre-aged at room temperature and cold rolled to reduce the thickness of the product and to increase its strength. Following cold rolling, the product is stretched to relieve residual stresses in the product. The stretching step is performed to flatten and strengthen the product and to remove residual quenching and/or rolling stresses from the product. Hyatt et al. discloses a maximum of 1% stretching for plate products since stretching beyond 2-3% causes increased incidence of breakage during the stretching process. Also, it is difficult to maintain desired levels of fracture toughness if the product is stretched more than 1%. Extrusions are stretched 1-3% as is normally required for all commercial alloys. Since extrusions are not cold rolled, they are in a relatively soft condition prior to stretching. As a result, extrusions generally are not susceptible to an increased incidence of breakage during stretching greater than 1%.
However, difficulties have been encountered in 2000 series aluminum alloys due to property losses such as decreased fracture toughness as a result of final plate stretching operations. In order to achieve desired strength levels, final stretching may be extended beyond the 1% value discussed in the Hyatt et al. patent to values up to 3.0%. The increased strength levels of the stretched plate, however, are accomplished at the sacrifice of fracture toughness. In fact, it may not be possible to obtain minimum fracture toughness levels at these increased strengths.
Accordingly, a need has developed to increase fracture toughness levels of these types of aluminum alloys while still maintaining satisfactory strength to weight ratios.
In response to this need, the present invention provides a method of improving aluminum alloy plate fracture toughness by delaying the final stretching operation following cold rolling.
The patent to Hyatt et al. does not teach controlling the time period between cold rolling and stretching. Moreover, Hyatt et al. does not recognize the improvements in fracture toughness as a result of delaying the stretching operation following cold rolling by a predetermined time period.